The invention relates to brake boosters and applies more particularly to the adjustment of the jump of such boosters.
A brake booster generally comprises a control rod which moves forwards when the vehicle driver actuates the brake pedal. This movement of the control rod is transmitted to a plunger which actuates boosting means. These boosting means generally comprise a three-way valve, the actuation of which makes it possible to break communication between a front chamber and a rear chamber of the booster and connect the latter chamber to the atmosphere. Because the front chamber is normally under a vacuum, a boosting force is thus exerted on the piston which separates the two chambers. The piston then moves forwards, thereby acting on a push rod serving for actuating the master cylinder of the brake circuit.
Conventionally, the piston serving for transmitting the boosting force to the push rod acts on the latter via a reaction disc made of a deformable material, such as an elastomer. At rest, there is a small play between the front end of the plunger and the reaction disc. When the vehicle driver begins to actuate the brake pedal, this small play allows the plunger to move forwards immediately in order to control the boosting means and ensure an immediate response of the brakes.
The effect of transmitting the boosting force exerted on the piston to the push rod via the reaction disc is to compress the peripheral part of the reaction disc axially in proportion to the boosting force. The result of this compression of the peripheral part of the reaction disc is a deformation of the central part of this disc towards the front face of the plunger. When the boosting force exceeds a particular threshold, the play initially present at rest between the reaction disc and the front face of the plunger is taken up, so that the plunger is in contact with the reaction disc and the latter returns to the brake pedal a reaction force representing the braking force exerted on the brakes of the vehicle. This well-known arrangement enables the driver to proportion the braking force which he exerts on the pedal as a function of the resistance which he encounters, this resistance increasing with this force.
It emerges from the foregoing explanation that the reaction at the pedal begins to occur only when the boosting force generated as a result of the actuation of the brake pedal exceeds a particular threshold. This threshold is called the "jump" of the booster. It is an important characteristic of the booster. In fact, although the existence of the jump is necessary to ensure an immediate response of the brakes when the pedal is actuated, the manufacturers of motor vehicles usually wish the value of this jump to remain within particular limits, so that the boost does not reach too high a value without an increase in the reaction at the pedal.
However, especially because of the production tolerances of the various component parts of the booster, there can be appreciable differences in the value of the jump from one booster to another. Likewise, the master cylinders intended to be actuated by the push rod of the booster themselves experience a spread of their characteristics, and the performances of a brakebooster/master cylinder assembly can therefore vary within an excessively wide range. Moreover, at the present time there is no practical means for controlling or adjusting the value of the jump of the booster, and consequently the above-mentioned requirements are not always satisfied, except by resorting to complex checks involving a high outlay.